Problem: As the swim coach at Covington, William selects which athletes will participate in the state-wide swim relay. The relay team swims $\frac{2}{3}$ of a mile in total, with each team member responsible for swimming $\frac{2}{15}$ of a mile. The team must complete the swim in $\frac{3}{5}$ of an hour. How many swimmers does William need on the relay team?
Explanation: To find out how many swimmers William needs on the team, divide the total distance ( $\frac{2}{3}$ of a mile) by the distance each team member will swim ( $\frac{2}{15}$ of a mile). $ \dfrac{{\dfrac{2}{3} \text{ mile}}} {{\dfrac{2}{15} \text{ mile per swimmer}}} = {\text{ number of swimmers}} $ Dividing by a fraction is the same as multiplying by the reciprocal. The reciprocal of ${\dfrac{2}{15} \text{ mile per swimmer}}$ is ${\dfrac{15}{2} \text{ swimmers per mile}}$ $ {\dfrac{2}{3}\text{ mile}} \times {\dfrac{15}{2} \text{ swimmers per mile}} = {\text{ number of swimmers}} $ $ \dfrac{{2} \cdot {15}} {{3} \cdot {2}} = {\text{ number of swimmers}} $ Reduce terms with common factors by dividing the $2$ in the numerator and the $2$ in the denominator by $2$ $ \dfrac{{\cancel{2}^{1}} \cdot {15}} {{3} \cdot {\cancel{2}^{1}}} = {\text{ number of swimmers}} $ Reduce terms with common factors by dividing the $15$ in the numerator and the $3$ in the denominator by $3$ $ \dfrac{{1} \cdot {\cancel{15}^{5}}} {{\cancel{3}^{1}} \cdot {1}} = {\text{ number of swimmers}} $ Simplify: $ \dfrac{{1} \cdot {5}} {{1} \cdot {1}} = {5} $ William needs 5 swimmers on his team.